1. Field of the Invention
The present invention relates to a signal reception apparatus for DS-CDMA communication system, particularly to a take receiver for combining multi-path signals with phase compensation.
2. Prior Art
A spread spectrum communication system absorbs attention due to its high frequency efficiency as the users of the land mobile communication steeply increases. Among various types of spread spectrum communication, a direct sequence code division multiple access (DS-CDMA) communication system is going to be standardized by an international committee of communication, mainly in the field of the mobile cellular radio and wireless LAN.
Usually, one signal transmitted causes a plurality of propagation signals passing through different paths with different path lengths. Since these signals cannot be coherently added, a multi-path fading occurs. In the DS-CDMA system, the multi-path signal are resolved and utilized by combining them.
FIG. 7(a) shows an example of the frame format in the DS-CDMA system. Each frame consists of a plurality of slots, for example, 6 slots. Each slot consists of a pilot symbol block and a information symbol block. Each of the pilot symbol blocks P1, P2, . . . , Pn has a predetermined number of symbols, for example, 4 symbols, and includes a predetermined symbol sequence. Each of the information symbol blocks I1, I2, . . . , In has a predetermined number of symbols, for example, 36 symbols. The pilot symbol blocks and information symbol blocks are arranged one after another so that each information symbol block follows one pilot symbol block.
Symbol blocks are modulated by QPSK information modulation, and modulated by BPSK spreading modulation or QPSK spreading modulation, then transmitted.
A composite code is formed by composing a short code with a length equal to the symbol duration and a long code with length equal to multiple symbol duration.
FIG. 7(b) shows a conventional rake receiver. The signal received by an antenna 101 is converted into a intermediate frequency signal by a high frequency receiver portion 102. An output of the portion 103 is divided by a divider 103 into two components of in-phase component (I-component) and quadrature component (Q-component) to be input to multipliers 106 and 107, respectively. A wave of a local frequency is generated by an oscillator 104. The wave is input directly to the multiplier 106, and is input through a phase shifter 105 for shifting the wave in phase by xcfx80/2 to the multiplier 107. The multiplier 106 multiplies the intermediate frequency signal from the divider 103 by the wave from the oscillator 104. An output of the multiplier 106 is processed by a low-pass filter 108 so that the I-component base band signal Ri is generated. The multiplier 107 multiplies the intermediate frequency signal from the divider 104 by the wave from the phase shifter 105. An output of the multiplier 107 is processed by a low-pass filter 109 so that the Q-component base band signal Rq is generated. The quadrature detection is performed.
The base band signal Ri and Rq are input to a complex matched filter 110 for multiplying the base band signal by I- and Q-components of PN code sequence supplied from a PN code generator 111. This is despreading. The I- and Q-components Di and Dq of the output of the matched filter 110 are input to a signal level detector 112, frame synchronization circuit 114 and a phase compensation portion 115.
The signal level detector 112 calculates the power of received signal Di and Dq. The signal power level is input to a multi-path selection portion 113 for selecting N paths, for example 4 paths, which are with higher power level of than others.
The frame synchronization circuit 114 receives an information of the path with maximal power from the multi-path selection portion 113 for detecting the head of the frame according to the symbol pattern of the pilot symbol block.
An output of the multi-path selection portion 113 is input to the phase compensation portion 115 which compensates the phase of the selected paths for example up to 4 paths. The outputs are synchronized and combined by a rake combiner 116. Using the output of rake combinent 116, a decision is made by data decision portion 117 and the information symbol is recovered.
As mentioned above, the phase of the despread received complex signal are compensated by the portion 115 according to the phase rotation of the known pilot symbol in the received signal. This is necessary for the coherent detection because the absolute phase is needed in the coherent detection.
FIG. 8 shows the phase compensation portion 115. The despread pilot symbol Di and Dq output from the complex matched filter 110 is input to a means 120 for extracting and averaging the phase error in Di and Dq.
A compensation signal is output from the means 120 to a phase compensation means 130. The means 130 multiplies the despread information symbol block by the compensation signal so as to compensate the phase of Di and Dq.
When a pilot symbol transmitted is expressed as a complex a=ai+j aq and the pilot symbol received is P=Pi+j Pq, xe2x80x9caxe2x80x9d and xe2x80x9cPxe2x80x9d are different only in phase xcex8 by ignoring the difference between the amplitudes, as shown in the formula (1).
P=Pi+jxc2x7Pq=(ai+jxc2x7aq)xc2x7ejxcex8xe2x80x83xe2x80x83(1) 
As shown in the formula (2), the phase of the pilot symbol P is extracted by multiplying xe2x80x9cPxe2x80x9d by a conjugate complex of xe2x80x9caxe2x80x9d. (Pi, Pq) of the pilot symbol is called xe2x80x9cphase vectorxe2x80x9d, hereinafter.                                                         ev              =                                                (                                                            P                      i                                        +                                          j                      ·                                              P                        q                                                                              )                                ⁢                                  (                                                            a                      i                                        +                                          j                      ·                                              a                        q                                                                              )                                                                                                        =                                                (                                                                                    P                        i                                            ·                                              a                        i                                                              +                                                                  P                        q                                            ·                                              a                        q                                                                              )                                +                                  j                  ·                                      (                                                                                            P                          q                                                ·                                                  a                          i                                                                    -                                                                        P                          i                                                ·                                                  a                          q                                                                                      )                                                                                                                          =                              ⅇ                j0                                                                        (        2        )            
The average phase error of the pilot symbol is expressed by the formula (3).                                                         E              =                                                1                  L                                ⁢                                                      ∑                                          k                      =                      1                                        L                                    ⁢                                      xe2x80x83                                    ⁢                                                            (                                                                        P                          i                          k                                                +                                                  j                          ·                                                      P                            q                            k                                                                                              )                                        ·                                          (                                                                        a                          i                          k                                                -                                                  j                          ·                                                      a                            q                            k                                                                                              )                                                                                                                                              =                                                P                  q                                +                                  j                  ·                                      P                    q                                                                                                          (        3        )            
Here, xe2x80x9cLxe2x80x9d is the total number of symbols included within one pilot symbol block. L=4 for example. The upper letter xe2x80x9ckxe2x80x9d is an ordinal number of the pilot symbol. (Ei, Ej) is called xe2x80x9cerror vectorxe2x80x9d, hereinafter.
Usually, the pilot symbol a=ai+j aq is with ai=(xe2x88x921, +1) and aq=(xe2x88x921, +1). So the multiplication in the formula (2) can be implemented by controlling positive and negative sign of the received pilot symbol. The phase error E in the pilot symbol block can be calculated by an adder. Therefore, the calculation is executed by a simple circuit.
There are two methods for phase compensation using the average phase error of the pilot symbol.
FIG. 9(a) shows the first method of phase compensation. The information symbols I1, I2 and I3 are compensated by the phase error vectors E(1), E(2) and E(3) just before the information symbols, respectively. This method can be called extrapolating compensation. The first pilot symbol blocks P1 and the first information symbol block I1 are representatively described.
A vector for compensating the phase error of the pilot symbol block in each path can be calculated by the formulae (4) to (6). The vector (Mi, Mq) for the compensation is called xe2x80x9ccompensation vectorxe2x80x9d, hereinafter.
M=Mi+jxc2x7Mq xe2x80x83xe2x80x83(4) 
Mi=Ei xe2x80x83xe2x80x83(5) 
Mq=Eq xe2x80x83xe2x80x83(6) 
The received symbol D=Di+j Dq is multiplied by a conjugate complex of the compensation vector M as shown in the formula (7) so that the information symbol block in the slot is compensated in phase. A compensated received signal is expressed by Dhat (D with a symbol like a hat).                                                                         D                ^                            =                                                (                                                            D                      i                                        +                                          j                      ·                                              D                        q                                                                              )                                ·                                  (                                                            M                      i                                        -                                          j                      ·                                              M                        q                                                                              )                                                                                                        =                                                (                                                                                    D                        i                                            ⁢                                              M                        i                                                              +                                                                  D                        q                                            ⁢                                              M                        q                                                                              )                                +                                  j                  ·                                      (                                                                                            D                          q                                                ⁢                                                  M                          i                                                                    -                                                                        D                          i                                                ⁢                                                  M                          q                                                                                      )                                                                                                          (        7        )            
Other paths selected by the portion 113 of the multi-path signal is similarly processed.
The rake combiner synchronizes the compensated signals and combines them. The output of the rake combiner is shown in the formulae (8) and (9). The output is expressed by Dbar (D with a symbol bar).                                           D            _                    i                =                              ∑                          n              =              1                        N                    ⁢                      xe2x80x83                    ⁢                                    D              ^                        i                          (              n              )                                                          (        8        )                                                      D            _                    q                =                              ∑                          n              =              1                        N                    ⁢                      xe2x80x83                    ⁢                                    D              ^                        q                          (              n              )                                                          (        9        )            
where, the superior (n) show the ordinal number of the path n=1, 2, . . . , N, and N=4 for example.
FIG. 9(b) shows the second method of phase compensation. The information symbols I1, I2 and I3 are compensated by pairs of the phase error vectors E(1) and E(2), E(2) and E(3), and E(3) and E(4) just before and behind each information symbol, respectively. This method can be called interpolating compensation. The first pilot symbol block P1 and the first information symbol block I1 are representatively described. When one pilot symbol block consists of 4 symbols, 36 symbols of one information symbol block is compensated by an average phase errors of 8 symbols of successive two pilot symbol blocks.
An error vector averaged over 4 pilot symbols is output from the means 120 as shown in the formulae (10) and (11).
E(t)=Ei(t)jxc2x7Eq(t) xe2x80x83xe2x80x83(10) 
E(t+1)=Et(t+1)+jxc2x7Eq(t+1) xe2x80x83xe2x80x83(11) 
The compensation vector M is calculated as in the formulae (12) and (13).
Mi=(Ei(t)+Ei(t+1))/2 xe2x80x83xe2x80x83(12) 
Mq=(Eq(t)+Eq(t+1))/2 xe2x80x83xe2x80x83(13) 
The mean between the averaged error vector E(t) just before the information symbol block Ii and the averaged error vector E(t+1) just behind Ii is used as a compensation vector M. The received vector D=Di+j Dq is multiplied by a conjugate vector of M so that Ii between E(t) and E(t+1) is compensated in phase.
The second method is higher accuracy than the first method, however a delay circuit such as a memory is necessary for holding one information symbol block until one phase error signal is calculated.
The path selection portion 113 calculates the electrical power from the despread signal as shown in the formula (14). An influence of interference and noise is included in the power.
|D|={square root over (Di2+L +Dq2+L )}xe2x80x83xe2x80x83(14) 
The power is always positive in despite of the despread signal. The peak value is not so much higher than other values when a plurality of symbols are averaged.
The present invention is invented so as to solve the conventional problems and has an object to provide a signal reception apparatus for DS-CDMA communication system.
According to the present invention, the signal reception apparatus calculates the power of phase corrected signal for reducing the influence of nosie and interference.